The present invention relates to a method of manufacturing a magnesium alloy member, which is a thixotropic material in which a solid material coexists with a liquid material.
A magnesium alloy member, which is excellent in light weight, high intensity, accuracy and fire retardancy and is a large-scaled thin member, can be enumerated as one of members which constitute the principal portion of a motor vehicle, an aircraft or the like. As technologies for shaping the member, an injection molding method for a thixotropic material, which is disclosed in Japanese Patent KOKOKU No. 33541/89 and Japanese Patent KOKOKU No. 15620/90, is known.
According to this injection molding method, athixotropic material such as a magnesium alloy having a dendrite structure is heated to a temperature in the range of from the liquidus temperature or more to the solidus temperature thereof or less in a molding machine so as to make a solid-liquid coexistent state; and a dendrite is sheared with a screw in the molding machine while the solid-liquid coexistent state is kept, so that the dendrite can be inhibited from growing until the dendrite is injected into a mold.
According to a method of casting a thixotropic material such as a magnesium alloy through an injection molding method, the granulation and growth of a dendrite are inhibited until the dendrite is injected into a mold. However, a thixotropic material such as a magnesium alloy is very high in thermal conductivity, and therefore, after the material is injected into a mold, it is quenched in the mold. This causes a rapid coagulation, which has been the main cause of the following problems.
That is, in the above injection molding method, the dendrite of the thixotropic material in a solid-liquid coexistent state at a temperature in the range of from the liquidus temperature or more to the solidus temperature or less in the mold is sheared and granulated so as to inhibit the growth. However, the thixotropic material exists in a solid-liquid coexistent state before it is injected into the mold, and thus there is a small difference between the temperature of the thixotropic material and the coagulation temperature thereof, which is commonly in the range of from 130xc2x0 C. to 160xc2x0 C. Therefore, the thixotropic material as injected into the mold begins to coagulate in a moment of time, whereby the flow pass of the thixotropic material in the mold rapidly becomes narrower. Hence, it is difficult to fill a mold for a thin shaped article, in particular, for a large-scaled complicated thin shaped product such as a motor vehicle with the thixotropic material to the end, and thus it is difficult to improve a large-scaled thin injection molded product in quality. In addition, since the flow pass of the thixotropic material in the mold rapidly becomes narrower, a liquid phase in the thixotropic material, which is easy to flow, escapes to the end of the mold, and/or can contribute to a molding sink, which makes the improvement of a large-scaled thin injection molded article in quality still more difficult.
Against the above-mentioned problems, countermeasures for keeping the temperature of a thixotropic material to the end of a mold have been taken. However, none of them has provided a solution for the above-mentioned problems.
For example, there exists a countermeasure, which comprises increasing the injection speed of a thixotropic material into a mold. That is, this countermeasure is intended to increase the injection speed of the thixotropic material into the mold for a large-scaled thin shaped product to five times or more as compared with the one in a resin injection molding method, or to 35 m/sec or more in some cases, so that the mold can be filled with the thixotropic material to the end in a minute range of temperature decrease. However, when the injection speed of the thixotropic material into the mold has been increased as mentioned above, a mold cavity and/or vortical traces on the surface of an injection molded product are often observed due to turbulence in the flow of the thixotropic material.
As another example, there exists a countermeasure, which comprises applying metal plating or coating of a heat insulating material to the surface of a mold. That is, metal plating or coating of a heat insulating material is applied to the surface over which a thixotropic material in the mold flows so that the heat insulating material can inhibit the temperature of the thixotropic material from decreasing when the thixotropic material is injected thereinto. In this case, the heat insulating material is largely different from a base material of the mold in coefficient of thermal expansion, and therefore, when a material which is heated to a high temperature of 500xc2x0 C. or more, with which the interior of the mold is filled, is repeatedly cooled in the mold, the plated metal or the coating of the heat insulating material is peeled in earliest stages, and thus the length of life is apt to be shortened. Furthermore, since the injection speed of the thixotropic material is rapid, the surface of the mold is intensely abraded by a solid portion of the thixotropic material, and there by the plated metal or the coating of the heat insulating material is worn away in earliest stages, whereby the life of the mold is further shortened.
Besides, it has been carried out to improve the flowability of a thixotropic material in a mold. For example, a material such as silica or potassium is added to a magnesium alloy so that a solid-phase particle of the magnesium alloy in a semi-molten state becomes minute and spherical so as to improve its flowability. However, with respect to this type of magnesium alloy, the improvement effect of flowability thereof is observed when the magnesium alloy is molded, while the material characteristics of the magnesium alloy member after molding molten, such as strength, cannot be improved.
Accordingly, the material characteristics of the magnesium alloy member after molding are generally inferior to those of an aluminum alloy member, and it has been said that it is difficult to improve the material characteristics thereof. For example, a magnesium-based magnesium alloy is largely weak in tensile strength and fatigue strength as compared with an aluminum-based aluminum alloy. As to tensile strength, the magnesium alloy has its strength of 230 Mpa, while the aluminum alloy has its strength of 315 Mpa. As to fatigue strength, the magnesium alloy has its strength of 70 Mpa, while the aluminum alloy has its strength of 130 Mpa.
Thus, as a countermeasure for increasing the strength of a magnesium alloy, carbon fibers have been used as a reinforcing material for magnesium alloy die-casting. That is, the carbon fibers and the magnesium alloy have been kneaded at a temperature of the solidus temperature or more (about 700xc2x0 C. or more) so that the magnesium alloy member can be reinforced with the carbon fibers. However, in this case, according to experimental results by the present inventors, as shown in FIG. 6 (which is a graph illustrating the relationship between xe2x80x9cthe content of C3Al4 in a carbon fiberxe2x80x9d and xe2x80x9cthe temperature of a molten Al liquidxe2x80x9d), when the carbon fiber and the magnesium alloy are kneaded at a temperature of 700xc2x0 C. or more, an aluminum component in the magnesium alloy reacts with the carbon fiber, whereby the carbon fiber becomes remarkably fragile, and thus it is difficult to improve the strength of a magnesium alloy member with the carbon fiber.
Furthermore, as a means for inhibiting the reaction of an aluminum component in a magnesium alloy with a carbon fiber whereby the carbon fiber is fragile when the magnesium alloy and the carbon fiber are kneaded at a temperature of 700xc2x0 C. or more, the surface of the carbon fiber is previously treated with metal plating or the like. However, it is difficult to treat the surface of a carbon fiber as described above from the viewpoint of a manufacturing process and capital investment, whereby the manufacturing cost of a magnesium alloy member becomes considerably high.
In addition, a material for a magnesium alloy member, the material being used for the present injection molding machine, is commonly in the shape of a chip, which is obtained by cutting an ingot of the magnesium alloy. In this chip-shaped material for the magnesium alloy member, when the ingot is cut, a cut powder, which is easy to ignite, arises therefrom, whereby the yielding percentage of the material may be decreased. Furthermore, in order to prevent a molten magnesium alloy in a mold from igniting, it is necessary to contrive cutting air off in a material hopper, which is freely imported together with the chip-shaped magnesium alloy material. However, this contrivance is difficult, in particular, when the member is continuously produced on a large scale, a lot of difficulty is involved.
For example, a manner of feeding a chip-shaped material for the magnesium alloy member into the material hopper (which is hereinafter referred to as xe2x80x9chopperxe2x80x9d) of the above injection-molding machine, and the problem thereabout to be solved will be explained.
A common feeding manner is the one in which the chip-shaped material for the magnesium alloy member (which is hereinafter referred to as xe2x80x9cchip materialxe2x80x9d) can be directly fed from a pouched device into the hopper. This feeding manner comprises the operation steps of: opening and closing the lid of the hopper while checking out the operations; and filling the interior of the hopper with an inert gas such as argon gas after closing the hopper. Thus it is very difficult to automate the operation steps.
Furthermore, another manner for feeding the chip material into the hopper is the one in which a system, as shown in FIG. 7, is used. This feeding manner is the one in which the chip material is continuously fed into a hopper (85) through a duct (83) with an air blower (81) from a material silo (82). In this manner, air is freely and continuously imported into the hopper (85), together with the chip material. Therefore, when the chip material is discharged into a barrel (84) of an injection-molding machine (87), a molten magnesium alloy is in danger of igniting, and thus it is necessary to shut off the interior of the hopper (85) from the air. Thus it is necessary to feed a lot of argon gas into the hopper (85) from an argon gas tank (86), or to make various complicated mechanical contrivances so as to prevent air from irrupting into the hopper (85). Consequently, the cost of facilities is increased.
Accordingly, a first object to be solved according to the present invention is to provide a method of manufacturing a magnesium alloy member, by which the shaping of a thin injection-molded member or the like for a motor vehicle or the like is facilitated, and the improvement of intensities thereof is facilitated, and furthermore the implementation thereof can be advantageously carried out in terms of capital investment.
According to the present invention, a magnesium alloy in which a carbon fiber is homogeneously dispersed is heated to a temperature in the range of from the solidus temperature or more to the liquidus temperature or less so as to obtain a solid-liquid coexistent magnesium alloy, wherein the carbon fiber has been cut into arbitrary lengths or powdered and has not been subjected to surface treatment; the above carbon fiber is homogeneously dispersed in the above solid-liquid coexistent magnesium alloy by a dispersion means so as to obtain a carbon fiber dispersed magnesium alloy; and then the above carbon fiber dispersed magnesium alloy is molded by means of a cylinder injection method or a die-casting method.
In addition, in the present invention, a series of the above operations is carried out in one selected from the group consisting of an inert atmosphere, a closed atmosphere, and a closed inert atmosphere. By manufacturing the magnesium alloy member in such a manner, it can be protected from deteriorating in quality due to oxidation.
Furthermore, in the present invention, the above solid-liquid coexistent magnesium alloy is dispersed by at least one means selected from the group consisting of agitation, subsonic vibration, shock wave vibration, and agitating vibration.
Besides, in the present invention, a magnesium alloy in which the content of the above carbon fiber is in the range of from 1 to 20% by weight, and the content of aluminum is 10% by weight or less is used as the above magnesium alloy.
According to the present invention as mentioned above, various operations and/or working-effects will be provided owing to the following technical reasons.
That is, as shown in FIG. 6 with respect to experimental results, the carbon fiber whose surface is not treated hardly reacts with an aluminum component at a temperature of 650xc2x0 C. or less at which the magnesium alloy is in a solid-liquid coexistent state, and thus even when the carbon fiber whose surface is not treated and the magnesium alloy are kneaded at a temperature of 650xc2x0 C. or less, the carbon fiber does not become fragile, and the strength of the carbon fiber is maintained, and the strength of the magnesium alloy member is drastically increased.
Furthermore, wetting properties of the carbon fiber whose surface is not treated to the magnesium alloy which is in a solid-liquid coexistent state are thoroughly suppressed because the surface of the carbon fiber is not treated, so that the carbon fiber can act as a barrier between molecules of the magnesium alloy which intensely move. Resultantly, the carbon fiber whose surface is not treated acts as a factor, which inhibits transmission of thermal energy in the magnesium alloy which is in a solid-liquid coexistent state, as well as a factor which inhibits the growth of a dendrite of the magnesium alloy because the carbon fiber has no wetting properties. Owing to these actions, the growth of a dendrite which is the largest problem to be solved in an injection molding method of a magnesium alloy which is in a solid-liquid coexistent state is retarded, and at the same time a rapid coagulation speed of the magnesium alloy in a mold is remarkably decreased.
Besides, as experimental data, Table 1 shows the tensile strength and the liquidity ratio of each of AZ91D which is one of conventional magnesium alloy members; a carbon fiber reinforced magnesium alloy member which is reinforced with a carbon fiber, which corresponds to a shaped product of the present invention; and a conventional aluminum alloy member.
Incidentally, liquidity ratios as shown in Table 1 were determined by comparing the inflow length of a material of the present invention and that of AZ91D, when each of the materials of the present invention and AZ91D was heated to an identical temperature in the range of from the liquidus temperature or more to the solidus temperature or less, and the material of the present invention was injected into a narrow and long tunnel through a injection molding machine, wherein the tunnel had had a temperature of 20xc2x0 C. and had been made of a mass of iron.
As can be apparently taken from Table 1, a carbon fiber magnesium alloy as reinforced with a carbon fiber whose surface is not treated may be delayed in growth of dendrite, and thus the fluidity is remarkably improved when the magnesium alloy is in a solid-liquid coexistent state. Resultantly, it is easy to fill the magnesium alloy to the end of a mold for a thin complicated shaped product without increasing an injection speed on molding to a large extent. Furthermore, it is unnecessary to largely increase a discharge pressure for increasing the injection speed, and thereby the leakage of the material from a gap of the mold is decreased, and thus it is easy to carry out secondary processing such as deburring after molding. Thereby, it is easy to manufacture a thin shaped product; in particular, it is easy to manufacture a large-scaled complicated thin shaped product, which has been conventionally considered to be difficult. With respect to a large-scaled thin shaped product, a shrinkage hole, an eddy vestige, a mold cavity or the like is inhibited from occurring. Thus the quality of a shaped product is remarkably improved.
Besides, as shown in Table 1, the carbon fiber magnesium alloy is remarkably increased in strength. This is because the carbon fiber was strongly fixed in the base material owing to an anchoring effect by which the base material of the magnesium alloy physically bites the surface of the carbon fiber that is not fragile.
In addition, as can be taken from Table 1, the magnesium alloy which is in a solid-liquid coexistent state hardly reacts with the carbon fiber, and thus surface treatment of a carbon fiber or precasting of a carbon fiber, which has been conventionally carried out in order to protect the carbon fiber from becoming fragile, is unnecessary. Furthermore, measures for elevating the temperature of a mold, coating of a heat-insulating material over the surface of a mold, or metal plating is unnecessary, and thus the drastically low cost of a mold and a mold with a long life can be realized.
Operations and/or working-effects owing to a carbon fiber whose surface is not treated as mentioned above depend upon the amount of the carbon fiber to a magnesium alloy, and the quality of material of the magnesium alloy itself. It is when a magnesium alloy has a carbon fiber content in the range of from 1% to 20% by weight ratio, and an aluminum content of 10% by weight ratio or less that the operations and/or working-effects as mentioned above are manifested; namely, when the content of the carbon fiber is less than 1% by weight ratio, the working-effects are small, while when the content is more than 20% by weight ratio, the quality of material of a magnesium alloy is deteriorated.
Besides, in the present invention, the shape of a material of a magnesium alloy member is intended to be in such one in which a wire or thin sheet shaped material is wound in the shape of a roll. It is effective for simplifying the steps of manufacturing a magnesium alloy member of the present invention, and for lowering the cost of a material for a magnesium alloy member that the shape of the material is specified. Furthermore, it is also effective for implementing the shutoff of air, which is most dangerous for the above material, when the material is fed into a hopper of an injection-molding machine, so that the shutoff can be advantageous from a viewpoint of capital investment.